Thursday, April 20, 2006

A very brief post. The new estimate for the cost of ongoing military operations in Iraq and Afghanistan is approaching $10B/month. For the foreseeable future. That's the entire NSF annual budget every 17 days. Remember that service on the national debt is also on the order of $20B/month. And the national debt is increasing at a rate of more than $2B/day. Some say to properly normalize those numbers you must remember that the GDP of the US is on the order of $12T/yr. Of course, it's also important to consider that 20% of all tax revenue in the US now goes to paying the debt service. How one can argue that this is all healthy is beyond me.

Monday, April 17, 2006

A second interesting discussion going on right now concerns hysteresis in molecular electronic device current-voltage characteristics. There have been a number of papers that have reported hysteretic IV curves in molecular systems, where applying a high bias can make the system undergo a transition from a low-conductance state to a higher conductance state. That higher conductance state persists until the bias voltage is cycled back down to some value below the turn-on voltage. This sort of hysteresis is interesting from the practical perspective: if the high/low conducting states are long-lived, one could imagine making a memory or logic out of devices with these properties. The main scientific question, then, is what is the underlying mechanism for this conductance switching?

One candidate that has been suggested by a number of people is polaronic. A polaron is a charge carrier accompanied by a geometric distortion of the charge carrying medium. The basic idea is that one can start with a neutral molecule, transfer an electron onto that molecule, and once that electron is there, the molecule could distort in such a way as to greatly lower the total energy of the system. The result is that the molecule could trap that additional electron via a geometric deformation. If the neutral and charged states of the molecule have significantly different couplings to the source and drain electrodes, this kind of trapping could conceivably lead to hysteretic switching between conductance states. Such a strong electron-vibrational coupling would basically make the effective on-site repulsion, U (for the no vibrational coupling case), be renormalized downward, all the way to a negative value.

This problem is interesting because it's fundamentally non-perturbative, at least in the electron-vibrational coupling, and generally non-equilibrium, too. Theorists have therefore been arguing about the right way to solve this system. As always, the whole point of this kind of theory is to come up with a toy model that includes all the essential physics and omits nothing of importance, and then use some method to solve it. If one leaves out the coupling between the electronic level and the leads, and considers just a single electronic level, this problem can be solved analytically, with no hysteresis showing up. One can include the coupling to the leads in some limit, and solve using Hartee-Fock techniques, again finding no hysteresis. One can choose a different set of limits, and find hysteresis; and finally, one can do a more sophisticated treatment of the nonequilibrium aspects and find telegraph-like switching rather than hysteresis. The big question is, which if any of these models are really relevant to the regime of experiments? It's highly likely that much switching in experiments really has to do with the geometry of the molecule-metal bond, rather than anything this exotic. Of course, that doesn't mean it's not worth trying to examine this question deliberately through experiments....

In the past couple of weeks, two interesting debates have come to my attention in condensed matter circles. The first has to do with electronic transport in graphene, and isn't really a debate - more of an interesting observation having to do with weak localization, a specific quantum correction to the classical electrical conductivity. Consider an electron propagating through a solid, scattering off of static disorder (lattice defects, grain boundaries). Feynman tells us that we have to add amplitudes for all possible paths through the material, and then square the sum of those amplitudes to get a transmission probability, assuming that all the paths add coherently. Some relevant trajectories include closed loops, that take the electron back past its starting point. For each loop like that, there is another trajectory with the loop traversed in the opposite direction. In the absence of spin-orbit scattering or magnetic fields, those loops and their time-reversed conjugates all add in phase and interfere constructively. The result is an enhanced (nonclassical) probability for the electron to back-scatter, leading to an enhanced resistance. Now, if one threads magnetic flux through those loops, electrons traversing loops in opposite directions are phase shifted relative to one another, and the constructive interference is broken. The weak localization enhancement of the resistance is suppressed at high magnetic fields, and the result is a magnetoresistance, with a field scale set by the size of the typical coherent loop. This is one of the main ways people estimate quantum coherence lengths in conductors.

What does any of this have to do with graphene? Well, here Andre Geim and coworkers look at transport in single graphene sheets, and find that weak localization is essentially absent. It turns out that the particular electronic structure of graphene implies that one can get the effect of a magnetic field if the graphene sheet isn't really flat. (For the experts: this has something to do with a pseudospin involving two equivalent sublattices on the sheet, and the breaking of that symmetry by roughness. I don't really understand this, so please let me know if there's a clear writeup about this somewhere.) Conversely, in a new paper de Heer and co-workers grow graphene epitaxially on SiC wafers, and do observe weak localization. Interesting - this seems to imply that the material grown by de Heer is in some ways intrinsically superior to that prepared by other methods. This is also roughly confirmed by the mobilities (25 m^2/Vs in de Heer's, 10 m^2/Vs in Geim's).

Tuesday, April 11, 2006

Since Science isn't going to run my letter to the editor, I'll just vent about it here. In last week's issue, Science ran a news article about the distressing tendency of retracted papers to linger on in the literature, sometimes still picking up citations long after the retraction. In the old days of strictly print journals, the excuse was that someone could stumble upon the original hardcopy of the retracted paper and not realize that it had been withdrawn. Now, though, the problem continues even in on-line versions of the papers. The Science reporters had expressed surprise that retraction notices don't always catch everyone's attention.

I find this very ironic, because Science has been part of the problem. Back in the dark days of late 2002, the Beasley Commission officially released their report, demonstrating beyond a shadow of a doubt that Jan Hendrik Schon was a complete fraud, and that his major papers needed to be retracted. The retractions happened almost immediately. Fast forward to December 2003, when two students writing final papers for my course mistakenly cite Schon's Science papers, despite their retraction over a year before. Why did the students not realize that the papers had been withdrawn? Because google had linked directly to the pdf versions of the papers, and Science had not marked up the pdf files to indicate the retraction. So, I used the on-line feedback form to tell Science about this problem. No response beyond an automated "Thank you for your email" formletter. Fast forward again to December, 2004. Again a student cites a Schon Science paper in the final paper for my course. Over two years after the fact, and the pdf files still don't indicate the retraction. I sent another letter, with a similar response.

Science has finally fixed this problem sometime in the intervening 15 months or so. I just find it funny that they seem to shift the blame onto their readership, when they themselves aggravated this problem by being too lazy to fix their pdf files for over two years. For Pete's sake - we're only talking about a handful of papers. It would've taken all of ten minutes to append the retraction to each file. Ahh well.

Sunday, April 09, 2006

It is strangely anticlimactic, and I think I know why. When you get your PhD, it happens at a well-defined moment. There's a defense, applause, a document that gets signed, etc. Tenure is much more diffuse. Months ago I submitted my "package" - my CV, some representative reprints, a statement of my research results and plans, etc. My department then sent out for external letters, and eventually had a vote of the tenured faculty on my case, as well as that of a couple of colleagues. The whole thing then got pushed forward to the dean's level, and eventually to the university's promotions and tenure committee. Fall turned to spring. Eventually I heard back positively, meaning that I got a letter telling me that in another month the board of trustees will give this there seal of approval, and then as of the next fiscal year (July 1), I'll be an associate professor. So you can see that the tenure transition is much more adiabatic, if you will. Day to day, nothing changes, though it's certainly nice!

Monday, April 03, 2006

Well, it's finally happened: my friends at Bell Labs are going to be learning to speak French, since Lucent and Alcatel are merging (as "equals", of course). What this means for Bell Labs is unclear. Since they do a fair bit of DOD work, at least part of the labs will have to be operated by an American-owned spinoff of some kind. This would further fragment the research organization, which was already split by the spinoff of Agere (motto: Welcome to Agere, Bell Labs researchers - here's your lay-off paperwork.) and the hemorrhaging of personnel, particularly in the physical sciences.

The continued shrinking of industrial research in the US is extremely depressing. There are things that can be done in an industrial research environment that just don't work well at a university. With the prevailing attitude that any research directed at long-term (say > 5 years) goals is effectively a waste of money unless it pumps up the stock price right now, it's no wonder that we're facing tough times in terms of competitiveness. I believe this is the equivalent of "eating the seed corn."

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About Me

My professional background: After an undergrad degree from Princeton in mechanical and aerospace engineering, I went to grad school at Stanford and got a doctorate in physics. Following a postdoctoral appointment at Bell Labs, I moved to Rice University and established a research program in experimental condensed matter physics, with a particular emphasis on nanoscale science. If you're interested in this stuff, please think about buying my book - it's a page-turner, and you'll want to finish it before the HBO miniseries spoils the ending. (That last part was a joke.) I blog regularly about science at Nanoscale Views. As should be obvious to pretty much everyone, anything I say there or here are my personal views, and in no way are official opinions of Rice University or its Department of Physics and Astronomy.